Methods for fabricating metal nanowires

ABSTRACT

Methods for the preparation of long, dimensionally uniform, metallic nanowires that are removable from the surface on which they are synthesized. The methods include the selective electrodeposition of metal nanowires at step edges present on a stepped surface, such as graphite, from an aqueous solution containing a metal or metal oxide. Where a metal oxide is first deposited, the metal oxide nanowires are reduced via a gas phase reduction at elevated temperatures to metal nanowires. Alternatively, beaded or hybrid nanowires comprising a metal A into which nanoparticles of a metal B have been inserted may be prepared by first electrodepositing nanoparticles of metal B selectively along step edges of a stepped surface, capping these nanoparticles with a molecular layer of an organic ligand, selectivley electrodepositing nanowire segments of metal A between nanoparticles of metal B and then heating the surface of the hybrid nanowire under reducing conditions to remove the ligand layer. In all three methods, the nanowires may be removed from the stepped surface by embedding the wires in a polymer film, and then peeling this film containing the embedded wires off of the stepped surface.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation of Ser. No. 09/976,990, whichclaims priority of Provisional U.S. patent application Ser. No.60/306,715 and Provisional U.S. Patent Application Serial No.60/317,862, which applications are fully incorporated herein byreference.

[0002] This invention was made with Government support under contractno. DMR-9876479. The government has certain rights in this invention.

FIELD OF THE INVENTION

[0003] The invention relates to metal nanowires and, more particularly,to methods which facilitate the fabrication of long, free standing metalnanowires.

BACKGROUND

[0004] Metal nanowires have great potential for immediate use in smallelectronic circuits, sensitive chemical sensors or any applicationrequiring metal filaments, and will likely be required as interconnectsin the nanometer-scale electronics of the future, especially thosefuture electronics which are not based on existing silicon technology.However, there are few methods for preparing nanowires that have suchtechnological utility. More particularly, few methods are capable ofproducing nanowires that are long (i.e., greater than ten microns inlength), uniform in diameter, free standing, and metallic. Two of themost successful approaches have been template synthesis and step-edgedecoration.

[0005] The template synthesis method, which is described in publicationsby the research groups of M. Moskovits¹⁶⁻¹⁸, C. R. Martin⁸⁻¹⁵, and P. C.Searson²¹⁻²⁷, appears to permit the growth of metal nanowires over awide range of diameters (from nanometer to micron scale) and for avariety of different metals. Template synthesis involves the growth ofcarbon, metals, or polymers in the void volumes of a nonconductiveporous host such as polycarbonate ultrafiltration membranes, porousAl₂O₃ films, and track-etched mica crystals all of which possess long(microns or longer), dimensionally uniform pores. In general, thesepores are oriented perpendicular to the plane of the porous film ormembrane. Nanowires are produced by filling these pores with aconductive material. Template synthesis, however, is limited by itsreliance on a template. Because all of the templates listed abovepossess linear, cylindrical (or prismatic) pores, only linear nanowirescan be produced. Moreover, nanowires produced by template synthesis arelimited to a maximum length that is equal to the thickness of the poroushost membrane, which tends to range from 0.1 to 20 microns.

[0006] The step edge decoration method, which is described inpublications by Himpsel³⁻⁴, Kern⁵⁻⁷, Behm² and others, involves theselective deposition of a metal or other material, such as CaF₂, atatomic step edges on a vicinal single crystal surface. Step edgedecoration can be controlled to yield continuous “wires” of varied widthand interwire spacing. Long nanowires that are many microns in lengthhave been prepared. Because the dimension of the “wire” perpendicular tothe vicinal surface has usually been limited to one or at most twoatomic layers, it has not been possible to remove these ultra thinmetallic ribbons from the surfaces on which they are synthesized. Thetechnological utility of such nanowires is necessarily limited as aresult.

[0007] In view of the foregoing, it would be desirable to providemethods that facilitate the fabrication of nanowires that are metallic,long (i.e., greater than ten microns in length), uniform in diameter,and removable from the surface on which they are synthesized and, thus,free standing.

SUMMARY OF INVENTION

[0008] The present invention is directed to methods that facilitate thefabrication of nanowires that are metallic, long (i.e., greater than tenmicrons in length), uniform in diameter, and removable from the surfaceon which they are synthesized and, thus, free standing. The fabricationprocesses of the present invention provide an electrochemical route todimensionally uniform and mechanically robust metal nanowires that rangein diameter from approximately 10-15 nm to approximately 1.0 μm, and areup to approximately 1.0 mm or more in length. The metal nanowiresproduced by the methods of the present invention advantageously exhibitwire diameter uniformity along the length of the wire as well as fromwire-to-wire for thousands of nanowires. Preferably, the relativestandard deviation from a mean diameter tends to be in a range of about5 to 20%, and more preferably is no more than about 10%. In addition,the metal nanowires produced by the methods of the present inventiontend to be electrically continuous along their entire length, i.e.,typically less than about 5% of the wires produced by the methods of thepresent invention tend to show any visible breaks.

[0009] The present invention includes three interrelated step-decorationmethods that are based on the selective electrodeposition of a metal ormetal oxide from an aqueous solution at step edges present on a basalplane-oriented surface, such as graphite, that is exposed to thesolution. In a first innovative aspect of the present invention (MethodI), metal oxide from an aqueous solution is selectively electrodepositedalong step edges present on the stepped surface to form precursor metaloxide wires. The metal oxide nucleates at a high linear density alongthe step edges forming beaded chains of metal oxide nuclei, which, withcontinued deposition, rapidly grow into a hemicylindrical nanowires.Wire growth involves low overpotential deposition at constant, or nearlyconstant, deposition current. Once formed, the precursor metal oxidenanowires are preferably reduced in hydrogen gas (H₂) at elevatedtemperatures to produce metal nanowires. Preferably this gas phasereduction occurs at about 500° C. for about an hour.

[0010] In a second innovative aspect of the present invention (MethodII), metal nanowires are prepared by direct electrodeposition of metalfrom an aqueous solution along step edges present on a stepped surfaceexposed to the solution. Following a nucleation pulse, the metalnucleates at a high linear density along the step edges forming beadedchains of metal nuclei, which, with continued deposition, grow intohemicylindrical nanowires. Wire growth involves low overpotentialdeposition at constant, or nearly constant, low deposition current. Thedeposition rates according to this method are preferably extremely low,e.g., the deposition rate to prepare a palladium nanowire via directelectrodepositon having a 200 nm diameter was about 10 minutes and a 300nm diameter was about 20 minutes.

[0011] In a third innovative aspect of the present invention (MethodIII), beaded or hybrid metal nanowires comprising a first metal (metalA) into which nanoparticles of a second metal (metal B) have beeninserted are prepared by first electrodepositing nanoparticles of metalB selectively along step edges of a step surface. The metal Bnanoparticles are then capped with a molecular layer of an organicligand having a strong affinity for the surface of the metal Bnanoparticles. Metal A is then selectively electrodeposited along thestep edges separating each metal B nanoparticle according to Method I orII. The ligand layer is preferably removed by heating the surface underreducing conditions in order to retain the metallic composition of theparticles and connecting nanowire segments. The nanoparticlesincorporated into the nanowires may range in diameter from the diameterof the nanowire itself, e.g., as small as about 10 nm, to about 1.0 μmor more.

[0012] In all three methods, the nanowires may be removed from thegraphite surface by embedding the wires in a polymer film, and thenpealing this film containing the embedded nanowires off of the graphitesurface. This removal step makes the nanowires available for electricalcharacterization, for use as interconnects for connecting twonanometer-scale elements of a circuit, for the implementation of avariety of nanowire-based devices including sensors and biosensors, andfor wiring of semiconductor quantum dots²⁰ to form a circuit. When usedin sensors, the binding of an analyte molecule to the surface of thenanowire induces a measurable change in the nanowire conductivity.

[0013] Other objects and features of the present invention will becomeapparent from consideration of the following description taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

[0014]FIG. 1 is a schematic diagram of Method I of the present inventionfor preparing metallic nanowires.

[0015]FIG. 2 is a schematic diagram of a nanometer scale electriccircuit incorporating an ensemble of metallic nanowires prepared inaccordance with the methods of the present invention.

[0016]FIG. 3 is a series of scanning electron microscope images ofmolybdenum nanowires prepared in accordance with Method I of the presentinvention.

[0017]FIG. 4 is a typical cyclic voltammogram for MoO₄ ²⁻ at a highlyoriented pyrolytic graphite electrode and scanning electron microscopeimages (above the cyclic voltommogram) of graphite surfaces obtainedfollowing the deposition of molybdenum dioxide (MoO₂).

[0018]FIG. 5 includes scanning electron microscope images of molybdenumnanowires embedded in a polystyrene film after removal from a graphitesurface.

[0019]FIG. 6 is a schematic diagram of Method II of the presentinvention for preparing metallic nanowires by direct electrodepositionof a metal.

[0020]FIG. 7 includes cyclic voltommograms for a graphite electrode intwo aqueous palladium plating solutions.

[0021]FIG. 8 is a graph showing the diameters of palladium nanowires asa function of the deposition time for nanowires deposited using theplating solutions indicated in FIG. 7.

[0022]FIG. 9 includes scanning electron micrographs of palladiumnanowires prepared by electrodeposition from aqueous solutions indicatedin FIG. 7.

[0023]FIG. 10 includes scanning electron micrographs of 300 nanometerdiameter palladium nanowires prepared in accordance with Method II ofthe present invention.

[0024]FIG. 11 is a schematic diagram of Method III of the presentinvention for preparing hybrid or beaded metal nanowires.

[0025]FIG. 12 is a scanning electron micrograph image of a graphitesurface following the implementation of the first three steps of MethodIII of the present invention.

DESCRIPTION

[0026] Referring in detail to the figures, methods that facilitate thefabrication of metallic nanowires and metallic nanowires prepared inaccordance with the methods of the present invention are shown. Themethods of the present invention involve fabrication processes thatprovide an electrochemical route to dimensionally uniform, mechanicallyrobust, long metal nanowires that are removable from the surface onwhich they are synthesized and, thus, free standing. The metal nanowiresproduced by the methods of the present invention tend to range indiameter from approximately 10-15 nm to approximately 1.0 μm and tend tobe greater than 10-20 μm in length and preferably hundreds of microns inlength on up to approximately 1.0 mm or more in length. The metalnanowires also advantageously exhibit high wire diameter uniformityalong the length of the wire as well as from wire-to-wire for thousandsof wires with a standard relative deviation from a mean diameterpreferably in a range of about 5 to 20% and more preferably no more thanabout 10%. In addition, the metal nanowires produced by the methods ofthe present invention tend to be electrically continuous along theirentire length, i.e., typically less than about 5% of the wires producedby the methods of the present invention tend to show any visible breaks.

[0027] The methods of the present invention include three interrelatedstep-decoration fabrication processes that are based on theelectrodeposition of a metal or metal oxide from an aqueous solution ona basal plane-oriented surface, such as graphite, that is exposed to thesolution. When suitable electric overpotentials are applied to theaqueous solution, the metal or metal oxide contained therein selectivelydeposits along the step edges present on the stepped surface forming“beaded-chains” of nuclei. With continued deposition, the beaded chainsform three-dimensional nanowires with diameters in a range of about10-15 nm to 1.0 μm for metal and in a range of about 20 nm to 1.3 μm fora metal oxide. The length of the nanowires tend to equal the length ofthe step edges on the stepped surface, which, with graphite inparticular, tend to be equal to the grain diameter.

[0028] A first embodiment of the present invention, i.e., Method I,shown schematically in FIG. 1, includes a first step (Step 1) whereinprecursor nanowires 14 are selectively electrodeposited along the stepedges 12 present on a stepped surface 10 from a dilute, preferablyalkaline (pH of approximately 8.5), aqueous plating solution. Theplating solution preferably includes an electrodepositable metal oxideat concentrations between about 1×10⁻³M and about 10×10⁻³M of the metalion of interest. The metal oxide in the plating solution tends tonucleate at an extremely high linear density, i.e., greater thanapproximately 20 nuclei/micron, along the step edges 12 forming “beadedchains” of metal oxide nuclei. With continued deposition, these beadedchains rapidly become smooth, hemicylindrical precursor nanowires 14. Asdeposited, the precursor nanowires 14 tend to be brittle andnonconductive, but are highly uniform in diameter, with diameters in therange of about 20 nm to 1.3 μm, and tend to be hundreds of microns toabout 1.0 mm or more in length.

[0029] Method I of the present invention includes a second step (Step 2)which involves a gas phase reduction of the precursor metal oxidenanowires 14 at elevated temperatures. Preferably, the metal oxidenanowires 14 are reduced in hydrogen gas (H₂) at about 500° C. for aboutone hour to produce metallic nanowires 16 that retain the dimensionaluniformity and hemicylindrical shape of the precursor, or “parent”,metal oxide composite nanowires 14. The metallic nanowires 16 tend to besmaller in diameter (about 10-15 nm to 1 μm) than the parent nanowires14 by about 30 to 35%, and tend to be mechanically resilient andelectronically conductive.

[0030] In a third step (Step 3) of Method I, the gas phase reduced metalnanowires 16, which tend to be only weakly associated with the steppedsurface 10, are embedded in a thin polystyrene film 18 that is cast ontothe nanowires 16 and the graphite surface 10. In a fourth step (Step 4)of Method I, the film 18, after it is allowed to air dry, is peeled offof the graphite surface 10 with the metal nanowires 16 embedded therein.The embedded nanowires 16 may comprise an ensemble of tens to hundredsof nanowires or more. The ensemble of nanowires 20, which have beenremoved from the graphite surface 10 and, thus, are free standing, mayadvantageously be incorporated into an electronic circuit or sensor 22as shown in FIG. 2. Low impedance electrical contacts 24 of silver,evaporated gold film and the like, may be connected to the ends of thenanowires 20.

[0031] Referring back to FIG. 1, the system used for electrodeposition,i.e. Step 1, preferably includes a glass electrochemical cell having avolume of approximately 50 mL. The plating solution noted above isintroduced into the cell along with three electrodes: A platinum“counter” electrode, a reference electrode (e.g., saturated calomelelectrode), and a “working” electrode, which is the surface, such asgraphite, on which the nanowires are to be grown. The two additionalelectrodes—i.e., the counter and reference electrodes—enable highprecision control of the potential of the working electrode. All threeelectrodes are preferably connected to a three-electrode potentiostat(e.g., EG&G Model 273A) which may be programmed to apply the requiredpotential to the working electrode.

[0032] The selective decoration of the step edges 12 and, thus, wiregrowth, in Step 1 occurs when the deposition is carried out at suitableoverpotentials, η_(dep) (where η_(dep)=E_(dep)−E_(eq)). Suitableoverpotentials, η_(dep), used in Step 1 for wire growth may range up toabout (−)900 mV versus the reversible potential¹, E_(eq), of thespecific material involved. If the deposition is carried out usinglarger overpotentials, nucleation tends to be spatially indiscriminantand metal oxide particles tend to be deposited everywhere on the surfaceof the step terrace 13. Moreover, if the overpotentials are too large,nucleation tends to occur on the surface of the step terrace 13 to theexclusion of the step edges 12.

[0033] The deposition process of Step 1 is preferably furthercharacterized by the application of a constant, or nearly constant,deposition current over the deposition period, which is typicallygreater than 20 seconds to grow nanowires of a desired size. Preferably,the constant deposition current is in a range of about 10 to 200microamps/cm² of electrode area. This rate invariance is consistent witha convection limited growth process where natural convective mixing ofthe electrolyte near an electrode surface occurs. Under these conditionsthe rate law for growth of a hemicylindrical solid becomes

r(t)=(2i _(dep) t _(dep) M/πnF _(ρ) L)^(1/2)  (1)

[0034] where r(t) is the radius of the hemicylindrical nanowire, i_(dep)is the deposition current, t_(dep) is the deposition time, M is theatomic weight of the deposited metal, n is the number of electronstransferred per metal atom, F is the Faraday constant, i.e., 96,485 Ceq⁻¹, ρ is its density, and L is the total length of the nanowire(s) onthe electrode surface. As indicated by Equation 1, the nanowire diameteris directly proportional to the square root of the deposition time. As aresult, nanowires of a particular diameter can be selectively producedby the methods of the present invention. Further, because dr/dt isproportional to t^(−1/2), the growth of highly dimensional uniformstructures, i.e., populations of nanowires that are narrowly dispersedwith respect to wire diameter, is possible.

[0035] As indicated above, the diameter of the precursor nanowires 14range from about 20 nm to 1.3 μm, which is typically many times theheight of the step edge 12 responsible for nucleating the growth of thewire 14. The height of the step edge 12 is typically about 0.3 to 2.0nm. Two factors tend to contribute to this “amplification” of the stepedge 12. First, at the low deposition potentials used in the methods ofthe present invention, the incipient nucleation sites tend to beconfined to the step edges 12 on the graphite surface 10, which helpsprevent the “spread” of the nanowire 14 onto terraces 13 during growth.The second factor is the inherent hemicylindrical symmetry ofdiffusional transport to metal nuclei arrayed along a linear step. Thenanowire 14 ends up with a hemicylindrical cross-section because theionic transport to the surface of the growing wire has this symmetry.These two factors operate in concert and permit the growth ofhemicylindrical wires with virtually any diameter from step edges havingmolecular dimensions.

[0036] It should be understood that Method I may be used to producemetal nanowires from any conductive metal oxide that iselectrodepositable, including MoO₂, Cu₂O, Fe₂O₃, and the like. Thefabrication of molybdenum nanowires is described below with regard toFIGS. 3-5 for exemplary purposes only. See also Zach et al., Science 290(2000) 2120²⁸, which is incorporated by reference as if set forth infull.

[0037] To prepare molybdenum nanowires in accordance with Method 1 shownin FIG. 1, molybdenum dioxide (MoO₂) is first electrodeposited (Step 1)according to the reaction:

MoO₄ ²⁻+2H₂O+2e⁻→MoO₂+4OH⁻  (2)

[0038] Under the conditions employed in Step 1, i.e., low overpotentialsin a range of about −600 mV to −800 mV and constant, or nearly constant,deposition current in a range of about 10 to 200 microamps/cm², MoO₂nucleates at an extremely high linear density (greater than about 20nuclei/μm) along the step edges 12. The deposited MoO₂ forms “beadedchains” of 5-8 nm diameter MoO₂ nuclei which, with continued deposition,rapidly become smooth, hemicylindrical nanowires. FIG. 3 shows a seriesof MoO₂ nanowires electrodeposited according to Step I of Method 1 alongstep edges of a stepped graphite surface. As shown in FIG. 3, the MoO₂nanowires include nanowires with diameters of 13 nm, 62 nm, 130 nm and210 nm. The numbers at the upper left indicate the deposition time, inseconds, i.e., 1s, 4s, 16s, and 256s, for preparing these MoO₂ nanowiresfrom an aqueous solution containing 1.0 mM MoO₄ ²⁻ by applying adeposition potential of about −0.75 V vs. SCE.

[0039] A typical cyclic voltammogram for MoO₄ ²⁻ at a highly orientedpyrolytic graphite electrode is shown in FIG. 4. As FIG. 4 indicates,the selectivity of MoO₂ nucleation is controlled by the depositionpotential. The scanning electron microscopy (SEM) images (above thecyclic voltammogram) of the graphite surfaces were obtained followingdeposition at potentials in a range of about (−)0.7 to (−)0.9 V versus asaturated calomel electrode (SCE) (right) and in a range of about(−)1.25 to (−)1.4 V versus SCE (left). As shown, electrodeposition ofMoO_(x), which has a reversible potential of about 0.1V, occurred with ahigh degree of selectivity at step edges at deposition overpotentials inthe range of (−)0.6 to (−)0.8 V versus SCE (right). The application of adeposition potential more negative than (−) 1.0 V (left) caused particlegrowth at surface terraces.

[0040] Once formed, MoO_(x) nanowires may be reduced in H₂ at 500° C. toproduce metallic molybdenum (Mo⁰) nanowires (Step 2). These metalnanowires are smaller in diameter by about 30% to 35% as compared withthe parent MoO_(x) wires, and are only weakly associated with thegraphite surface 10. Removal of these reduced wires from the graphitesurface is accomplished by embedding the nanowires in a polystyrene film(Step 3), and lifting this film together with the embedded nanowires offthe graphite surface (Step 4). FIG. 5 provides SEM images of molybdenumnanowires embedded in a polystyrene film after removal from a graphitesurface. The ability to remove molybdenum nanowires from the graphitesurfaces on which they are synthesized facilitates the technologicalutility of these nanostructures both in electronic devices, asinterconnects, and in sensors.

[0041] Turning to FIG. 6, a second embodiment of the present invention,Method II, is shown schematically to involve the preparation of metalnanowires by “direct” electrodeposition of a metal on a stepped surface110, such as graphite, that is exposed to an aqueous solution containingthe metal. In a first step (Step 1), nanowires 116 are selectivelyelectrodeposited along the step edges 112 present on a stepped surface110 from an aqueous plating solution comprising a electrodepositablemetal. The solution preferably includes metals such as palladium, goldand the like, at concentrations between about 1×10⁻³ and 10×10⁻³M of themetal ion of interest. Electrodeposition of gold, however, is preferableperformed in an electrochemical cell that is pressurized to about 40atm. Following a nucleation pulse, the metal in the plating solutionnucleates at an extremely high linear density (i.e., greater than about20/μm) along the step edges 112 forming “beaded chains” of metal nuclei,which, with continued deposition, become smooth, hemicylindricalnanowires 116.

[0042] Preferably, the electrodeposition is carried out at very lowdeposition overpotentials of up to about (−)400 mV and preferably in arange of about (−)10 to (−)200 mV. To increase nucleation density and,thus, ensure that the nanowires are continuous, a nucleation pulse, wellnegative of the reversible potential, may be applied for about fivemilliseconds prior to electrodeposition at the desired overpotential.The deposition is preferably carried out at low constant, or nearlyconstant, deposition current, e.g., preferably less than 50 mA/cm².Depending on the metal being deposited and the applied current density,which is preferably in a range of about 5 μAcm⁻² to 50 μAcm⁻², thedeposition rates in Method II are preferably extremely low. For example,as shown in FIG. 8 and discussed below, the deposition time to prepare apalladium nanowire having a 200 nm diameter was about ten minutes.Deposition rates to prepare metal nanowires of a desired diameter may bedetermined according to Method II may be calculated using Equation (1).

[0043] In a second step (Step 2) of Method II, the metal nanowires 116are embedded in a thin polystyrene film 118 that is cast onto thenanowires 116 and the graphite surface 110. In a third step (Step 3),the film 118, after it is allowed to air dry, is peeled off of thegraphite surface 110 with the metal nanowires 116 embedded therein. Theembedded nanowires 16 may comprise an ensemble of tens to hundreds ofnanowires or more. The ensemble of nanowires, which have been removedfrom the graphite surface 110, may also advantageously be incorporatedinto an electronic circuit or sensor 22 as shown in FIG. 2.

[0044] Palladium nanowires prepared in accordance with Method 11 aredescribed in regard to FIGS. 6-10 for exemplary purposes only. Palladiumnanowires may be electrodeposited from aqueous solutions containingpalladium. Examples of such solutions include 2.0 mM Pd²⁺, 0.1 M HCl,water, and 2.0 mM Pd²⁺, 0.1 M HClO₄, and the like. Palladium nanowiresprepared by the process of Method II are shown in FIGS. 9-10. Startingwith a freshly cleaved graphite surface within a palladium platingsolution, these nanowires were prepared by first applying a 5 msnucleation pulse of −0.2 V (vs. saturated calomel electrode, SCE). Asshown in FIG. 7, this potential is well negative of the reversiblepotential for palladium deposition in these solutions (+0.6 to +0.7 Vvs. SCE). After this nucleation pulse, the growth of palladium nanowireswas carried out using potentials in the ranges shown in gray in FIG. 7.These deposition potentials produce deposition current densities rangingfrom about 30-60 μA cm⁻² and deposition times for 200 nm diameter wiresof about 10 minutes (see FIG. 8). The deposition times for palladiumnanowires having 300 nm diameters, as shown in FIG. 10, were about 20minutes.

[0045] The morphology of the palladium nanowires, as well as other metalnanowires, obtained by electrodeposition tends to be dependent on theidentity of the electrolyte present in the plating solution. Forexample, palladium nanowires deposited from HCl solutions, as shown inFIG. 9 (left), tend to be rough and granular. The dimensions of thegrains in these polycrystalline wires as estimated from SEM imagesranged from about 50 to 300 nm. Continuous nanowires of 150 nm indiameter have been obtained from this solution. Deposition of palladiumnanowires from HClO₄ solutions as shown in FIG. 9 (right), yieldnanowires having a smoother morphology. The grains in these nanowireswere 10-50 nm in diameter. A smoother morphology permits nanowires asnarrow as 55 nm in diameter to be deposited. The rough and smoothnanowires prepared using these two plating solutions behave electricallyidentical to one another.

[0046] A third embodiment of the present invention, Method III, involvesthe preparation of beaded or hybrid metal nanowires comprising a firstmetal (metal A) into which nanoparticles of a second metal (metal B)have been inserted. These hybrid metal nanowires are prepared, as shownschematically in FIG. 11, by first (Step 1) electrodepositingnanoparticles 215 of metal B selectively along step edges 212 of astepped surface 210 such as graphite. The metal B nanoparticles 215,which are preferably formed from a noble metal including nickel,palladium, platinum, gold, and the like, are electrodeposited, e.g.,from an aqueous solution comprising 1.0×10⁻³ m to 10×10⁻³ m of the metalion of interest using a suitable overpotential. Platinum nanoparticles,for example, are preferably deposited for 100 ms from a 1.0×10⁻³ mpt^(2t) solution using an overpotential of −0.5V in order to obtain 10nm diameter metal nanoparticles at a density of about 10⁸ to 10¹⁰ cm⁻².See, e.g., Zach et al., Adv. Mat., 12 (2000) 878²⁹ and Zoval et al., J.Phys. Che. B. 102 (1998) 1166³⁰, and others which are incorporated byreference as if set forth in full.

[0047] The deposited metal B nanoparticles 215 are then (Step 2) exposedto an ehanolic solution of an aklane thiol. As a result of the thiolexposure, each nanoparticle is “capped” by a self assembled molecularmonolayer of an organic ligand 217 having a strong affinity for thesurface of the metal B nanoparticles 215. Examples of such lygandsinclude Thiols (chemical formula: R—SH where R is a hydrocarbon), whichhave an affinity for noble and coinage metals including Pt, Pd, Au, Ag,and Cu, and Nitriles (chemical formula: R—CN where R is a hydrocarbon),which have an affinity for Pt, Pd and Ag.

[0048] In a next step (Step 3), a metal A or a metal A oxide isselectively electrodeposited along the step edges 212 separating eachmetal B nanoparticle 215 according to Method I or II discussed above toform a metal A or metal A oxide nanowire 214, 216 between the metal Bnanoparticles 215. Because the ligand 217 forms an electricallyinsulating layer atop of the metal nanoparticles 215, the deposition ofthe wire material does not occur on top of the nanoparicles 215, justbetween the nanoparticles 215.

[0049] In a final step (Step 4), the ligand layer 227 is preferablyremoved by heating the surface under reducing conditions in order toretain the metallic composition of the particles 215 and connectingnanowire segments 216. Preferably, the reduction of the surface occursin H₂ at 500° C., which results in the alkane thiol being pyrolyaed. Thenanoparticles incorporated into the nanowires may range in diameter fromthe diameter of the nanowire itself, e.g., as small as about 10 nm, toabout 1.0 μm or more. FIG. 12, which is a SEM image of a graphitesurface following Step 3 of Method III, shows hybrid nanowirescomprising nickel nanoparticles and molydbenum dioxide nanowire segmentsprepared according to Method III.

[0050] Like the metal nanowires prepared according to Methods I and II(see FIGS. 1 and 6), the hybrid nanowires prepared according to MethodIII may be removed from the graphite surface by embedding the wires in apolymer film, and then pealing this film containing the embeddednanowires off of the graphite surface. Because the hybrid nanowires areremovable they may be utilized as elements of nanometer-scale circuits,sensors, biosensors, and the like. When used in sensors, the nanowiresegments are employed to “read-out” any measurable change in thenanoparticle conductivity induced by the binding of an analyte moleculeto the surface of the nanoparticle.

[0051] While the invention is susceptible to various modifications andalternative forms, specific examples thereof have been shown in thedrawings and are herein described in detail. Many alterations andmodifications can be made by those having ordinary skill in the artwithout departing from the inventive concepts contained herein. Itshould be understood, therefore, that the illustrated embodiments havebeen set forth only for the purposes of example and that they should notbe taken as limiting the invention.

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What is claimed is:
 1. A method for preparing metallic nanometer scalewires comprising the steps of electrodepositing a metal at a step edgepresent on a stepped surface to form a wire, and removing the wire offof the stepped surface.
 2. The method of claim 1 wherein theelectrodepositing step includes electrodepositing metal from an aqueoussolution including a desired metal.
 3. The method of claim 2 wherein theelectrodepositing step includes applying a nucleation pulse to theaqueous solution.
 4. The method of claim 3 wherein the electrodepositingstep further includes applying a deposition potential to the aqueoussolution.
 5. The method of claim 4 wherein the over potentialcorresponding to the deposition potential is less than about −400millivolts.
 6. The method of claim 1 further comprising the step ofembedding the wire in a film of material.
 7. The method of claim 6wherein the film of material comprises a polymer.
 8. The method of claim6 wherein the removing step includes lifting the film and embedded wireoff of the stepped surface.
 9. A method for preparing metallic nanowirescomprising the steps of electrodepositing metal oxide at step edgespresent on a stepped surface to form a metal oxide wire, reducing themetal oxide wire to a metal wire, and removing the metal wire from thestepped surface.
 10. The method of claim 9 wherein the electrodepositingstep includes electrodepositing metal oxide particles from an aqueoussolution including a desired metal oxide.
 11. The method of claim 10wherein the electrodepositing step includes applying a depositionpotential to the solution.
 12. The method of claim 11 wherein the overpotential corresponding to the deposition potential is less than −900millivolts.
 13. The method of claim 9 wherein the reducing step includesreducing the metal oxide wire to a metal wire via gas phase reduction.14. The method of claim 13 wherein the reducing step includes reducingthe metal oxide wire in hydrogen gas.
 15. The method of claim 13 whereinthe reducing step includes reducing the metal oxide wire at about 500°C.
 16. The method of claim 9 further comprising the step of embeddingthe metal wire in a film of material.
 17. The method of claim 16 whereinthe film of material comprises a polymer.
 18. The method of claim 16wherein the removing step includes lifting the film and embedded wireoff of the stepped surface.